The present invention relates generally to semiconductor power devices, and more particularly to a semiconductor power device such as a MOSFET and other power devices that use floating islands of oppositely doped material to form the voltage sustaining layer.
Semiconductor power devices such as vertical DMOS, V-groove DMOS, and trench DMOS MOSFETs, IGBTs as well as diodes and bipolar transistors are employed in applications such as automobile electrical systems, power supplies, motor drives, and other power control applications. Such devices are required to sustain high voltage in the off-state while having low on-resistance or a low voltage drop with high current density in the on-state.
FIG. 1 illustrates a typical structure for an N-channel power MOSFET. An N-epitaxial silicon layer 101 formed over an N+ doped silicon substrate 102 contains p-body regions 105a and 106a, and N+ source regions 107 and 108 for two MOSFET cells in the device. P-body regions 105 and 106 may also include deep p-body regions 105b and 106b. A source-body electrode 112 extends across certain surface portions of epitaxial layer 101 to contact the source and body regions. The N-type drain for both cells is formed by the portion of N-type epitaxial layer 101 extending to the upper semiconductor surface in FIG. 1. A drain electrode is provided at the bottom of N+ doped substrate 102. An insulated gate electrode 118 comprising insulating and conducting layers, e.g., oxide and polysilicon layers, lies over the body where the channel will be formed and over drain portions of the epitaxial layer.
The on-resistance of the conventional MOSFET shown in FIG. 1 is determined largely by the drift zone resistance in epitaxial layer 101. Epitaxial layer 101 is also sometimes referred to as a voltage sustaining layer since the reverse voltage applied between the N+ doped substrate and the P+ doped deep body regions is sustained by epitaxial layer 101. The drift zone resistance is in turn determined by the doping concentration and the thickness of epitaxial layer 101. However, to increase the breakdown voltage of the device, the doping concentration of epitaxial layer 101 must be reduced while the layer thickness is increased. The curve in FIG. 2 shows the on-resistance per unit area as a function of the breakdown voltage for a conventional MOSFET. Unfortunately, as the curve shows, the on-resistance of the device increases rapidly as its breakdown voltage increases. This rapid increase in resistance presents a problem when the MOSFET is to be operated at higher voltages, particularly at voltages greater than a few hundred volts.
FIG. 3 shows a MOSFET that is designed to operate at higher voltages with a reduced on-resistance. This MOSFET is disclosed in Cezac et al., Proceedings of the ISPSD, May 2000, pp. 69-72, and Chen et al., IEEE Transactions on Electron Devices, Vol. 47, No. 6, June 2000, pp. 1280-1285, which are hereby incorporated by reference in their entirety. This MOSFET is similar to the conventional MOSFET shown in FIG. 1 except that it includes a series of vertically separated Pxe2x88x92 doped layers 3101, 3102, 3103, . . . 310n (so-called xe2x80x9cfloating islandsxe2x80x9d), which are located in the drift region of the voltage sustaining layer 301. The floating islands 3101, 3102, 3103, . . . 310n produce an electric field that is lower than for a structure with no floating islands. The lower electric field allows a higher dopant concentration to be used in the epitaxial layer that in part, forms the voltage sustaining layer 301. The floating islands produce a saw-shaped electric field profile, the integral of which leads to a sustained voltage obtained with a higher dopant concentration than the concentration used in conventional devices. This higher dopant concentration, in turn, produces a device having an on-resistance that is lower than that of a device without one or more layers of floating islands.
The structure shown in FIG. 3 can be fabricated with a process sequence that includes multiple epitaxial deposition steps, each followed by the introduction of the appropriate dopant. Unfortunately, epitaxial deposition steps are expensive to perform and thus a structure that uses multiple epitaxial deposition steps is expensive to manufacture.
Accordingly, it would be desirable to provide a method of fabricating a power semiconductor device such as the MOSFET structure shown in FIG. 3, which method requires a minimum number of epitaxial deposition steps so that the device can be produced less expensively.
In accordance with the present invention, a method is provided for forming a power semiconductor device. The method begins by providing a substrate of a first conductivity type and then forming a voltage sustaining region on the substrate. The voltage sustaining region is formed by depositing an epitaxial layer of a first conductivity type on the substrate and forming at least one terraced trench in the epitaxial layer. The terraced trench has a plurality of portions that differ in width to define at least one annular ledge therebetween. A barrier material is deposited along the walls of the trench. A dopant of a second conductivity type is implanted through the barrier material lining the annular ledge and said trench bottom and into adjacent portions of the epitaxial layer. The dopant is diffused to form at least one annular doped region in the epitaxial layer. One other region located below the annular doped region may also be formed. A filler material is deposited in the terraced trench to substantially fill the trench, thus completing the voltage sustaining region. At least one region of the second conductivity type is formed over the voltage sustaining region to define a junction therebetween.
The power semiconductor device formed by the inventive method may be selected from the group consisting of a vertical DMOS, V-groove DMOS, and a trench DMOS MOSFET, an IGBT, a bipolar transistor, and diodes.